Optical system



3 W. l .w j m a e 1. wf 0 v .H j. m .wf m W r O 5 7 r T mm. W WF1 m w M 2,4 2 S ?1 ,www A mw 9 .f (br U ffw@ F, f. 4, O

Patented Oct. 4, 1938l UNITED STATES PATENT orrlcE t OPTICAL SYSTEH Archer vHoyt, Aspinwall, Pa., assignorto Gul! Rcsearch &

Development Company, Pittsburgh.

Pa., a corporation of Delaware Application March 30,

193e, serai n. 11.131

11 Claims. v (CL 88-29 'I'his invention relates to optical systems; and it comprises optical means for indicating angular 'deilection of an angularly movable apparatus part, comprising a pair of plane, partially trans- "-\parent mirrors arranged substantially parallel to each other, one mirror being fixed and one atftached to said apparatus part, so that upon movement of said part the mirror attached thereto 7 is tilted with respect to the fixed mirror, a source of light, means for directing a collimated beam of iightfrom the source throughthe mirrors substantlally at right angles to the plane of the mirindicating system which rors, and means for bringing the collimated beam to a focus at a focal piane after its passage through the mirrors, so that upon movement of the apparatus part. inter-reections occur between the mirrors and a series of images of the source' is formed at the focal plane; all as more fully hereinafter set forth and s claimed.

Many physical measuring instruments indicate the measurement as an angular deflection. For example, there has Ibeen developed a gravimeter.. an apparatus for measuring the force of gravity at any given point on the surface of the earth. which indicates changes in gravity as an angular deflection of a portion of the apparatus.

In instruments where the angular deection is large or where great sensitivity is not required the deflection may be indicated by a simple needle or pointer attached to the moving part. In more delicate instruments it is usual to employ in lieu of a metal pointer, a so-calied light beam pointer. That is, a mirror is attached to the moving part. and arranged to reiiect a beam of light from a iixed source to a iixed screen. Upon any twisting of the moving part, a spot of light moves over the screen. arrangement presents advantages over material pointers in that the pointer is merely a beam of light which has no mass or friction; and also that the angular deflection indication is, through optical lever eiect. amplified by a factor of 2. However. this single reflection arrangement. while an improvement upon mechanical pointers. has limitations. To secure great sensitivity it has been found necessary to employ light-paths six feet long or more. which makes for a bulky and cumbersome appatus.

One object of the invention is to provide an optical indicating system which utilizes a light beam as a pointer but which gives an enhanced amplification of the angular deflection.

Another object is the provision of an optical is of great compact- Another object is the provision of such a system utilizing multiple reflections to secure amplification and in w ch the exact order of redection used is apparent to the observer.

Another object sthe provision of such a'system adapted for measurement of very small angular deflections.

Another object is to provide an optical system which is very sensitive but which is rugged, easy to adjust. and capable of being operated by unskilled persons.

These objects are achieved by the provision of an optical indicating system which includes, es-

sentially, a projection lens combination and two partially transparent mirrors at ornear the cen 1 ter of thecombnation, one of the mirrors being fixed and the other moving. The mirrors may be combined with the lenses or may be separate elements. The moving mirror is attached to some angularly-defiectable part of the measur- 20 ing apparatus to which the system is applied. A source of light and a screen are set up in such manner that the s ource is imaged on the screen by the lens combination. Then uponangular deflection of the moving element, a series of multiple reflection images appears on the's'creen, each of which images indicates the actual angular deflection multiplied by a different factor. The principles of operation will be clear in the extended descrlption to follow. l

The new system will be described in three typical adaptations: to a gravimeter, an analytical balance and a barograph. Two principal modifications of the system itself will be described, to illustrate the invention. Examples of specific embodiments of the invention are illustrated in the accomp ng drawings, in which Fig. l is a simplified view, partly in elevation and partly in vertical section, of one embodiment. of the invention incorporated in a gravimeter;

Fig. 2 is a diagrammatic view of the optical system oi Fig. 1:

Fig. 3 is an optical diagram illustrating the operation of the optical system of Figs. l and 2;

Fig. 4 shows an advantageous form of slit:

Fig. 5 shows what is seen in the eyepiece when the slit of Fig. 4 is used;

Fig. 6 is an optical diagram of a modification of the system shown in Fig. 2:

Figs. 'i and 8 are diagrammatic views in two different directions. of a modified form of optical system:

Figs. 9. l0 and il show modified forms of lens combinations:

Fig. l2 shows an analytical balance incorporating the optical system;` Fig. 13 is a fragmentary sectional view of a portion of Fig. 12;

Fig. 14 shows a barographvincorporating they optical system; n Fig. 15 shows a modication giving echelon images: and A Fig. 16 shows the image array with the system of 1518.15. j

In the: showings, in which like reference characters indicate like 'parts throughout. 1 shows one embodiment incorporated in a loadedspring gravimeter. The gravimeter, of which the showing is simpliiied, comprises a casing 2l with top 2l and base 22. A helical coil spring 23 is provided,A being suspended from lthe casing top by a post 2l and angularly-adjustable plug 25, and having attached to its lower end a post 26 carrying an annular weight 2l. The spring is oa type adjusted to twist imder the influence of variations in weight of .the annulus 21, and hence to impart a twisting movement to post 26, reecting changes in the earth's gravity. The twist is in all cases of very small magnitude. making special indicating means advisable for securing accuracy. The gravimeter, including the spring,is described in` detail in my copending application Serial No. 34,824, filed Augusti'i,l 1935.

The optical system of the present invention is' to indicate angular deections of post 265 As shown, lthere vis aflixed to post 26 an annular mount 3|! retaining a. lens-mirror 3i having 'a convex unsilvered vface 32 and a plane face partly silvered as at 32 forming a mirror. A similar lens-mirror 3l having corresponding faces 22 and 33 is mounted on the base (22) by means of an annular mount 3B. The lenses vare ordinarily made of glass, though they can be made of Iother ytransparent material if desired.

' 'I'he plane faces 33 of the lenses are parallel, in

the normal or Azero position of the apparatus. If necessary knob. 25 vis adjusted to lbring the faces into parallelism. The plane faces are arranged closely adjacent. vIn top 2i is attached a housing 31 containing a light bulb 38, a condensing lens 39 and a slit element Il (Fig. 4) having a slit 4I. The slit member can be of the adjustable type if desired and can be of 'shape other than that shown. yAlso mounted in the top is an eyepiece l2 including a lens 43 and a transparent screen member 4I (Fig. 5) having a scale I5. In Figs. 2 and 6, 7 and 8 the eyepiece is indicated diagrammatically at l2, the appropriate image ray lines for one of the images (thev undeilected image) being indicated at 45. In lieu of a screen and scale. the observing means may comprise a micrometer eyepiece with crosshairs, or ocular observing means may be replaced by recording means such as a piece of photographic material (described post). The term screen is used herein to include the focal plane of the eyepiece and any sort of index or receiving surface, upon which the indicating images are ioc In lieu ot a slit. a narrow linear lamp filament or a point source can be used. as described post. Two degree prisms 50 having reflecting hypotenuse faces 5I are mounted on the base in supportsv 52. The function of the prisms is to direct light from the slit through the. lenses and back up to the eyepiece.

The operation of the optical system is best seen in Fig. 2, which shows the several elements thereof, except the prisms, which are omitted since their only function is to change direction 'of the .light beams. The image of the illament of bulb 38 is focused between lenses 3i and 3l and thus the slit isilluminated substantially uniformly. The illuminated slit serves as the eiiective source of light for the system. The lenses, slit and scalev are so arranged that the slit is imaged on the scale, as shown. In all embodiments of the invention it is provided that the light beam in the inter-lensl space is collimated; i. e., the rays are parallel. In the arrangement of Fig. 2 this condition is achieved by placing the scale and the slit at the principal foci oi' the lens combination. When the lenses are parallel the slitis imaged. onthe scale at BU. This is the normal or zero position of the system. Upon angular deection of the post 2l the lenses are put out of parallel. Inter-reflections between the plane mirror faces occur, the phenomenon being diagrammed in Fig. 3. Each inter-reilected bundle of rays gives rise to an image on the screen, as indicated at 8|, 62, 62, GI and 65. A large number of reection images are produced; only ve are shown. The appearance of the screen and scale in theeyepiece is indicated in Fig. 5, the undeviated direct image and the higher order images appearing as shown. v

In all cases the moving lens should be so mounted on the post that the nodal axis of the lens coincides, or approximately coincides, with the axis of rotation. In lplano-convex lenses 'such as those shown, the nodal axis is tangent inter-reflections represents the angular displacement of post 26, angularly amplified by a factor depending on the lnumber of inter-reflections responsible for that image. Thus the first interreilection noted at 66 on Fig. 3 gives rise to a ray 61 which produces image 6| and which makes an angle 2A equal to twice the angle A through which post 26 moves; The second inter-redection noted at 68 gives rise to a ray s'which produces image 62 and which makes an angle IA. Higher orders of inter-reections give rise to ray: making an angle which is equal to the angle oi' deiection of post 2B multiplied by twice the number of reectlons. That is, the 11th image from the. undeiiected image moves through an angle 211A when post 26 moves through an angle A. VIn Fig. 3 all the angles are shown exaggerated for the sake of clarity.

The observer may select any of the images. For example. he may select the 6th image, and note that the scale reading is 15 millimeters from the zero image to the 6th image. Scale readings are suiicient ior most purposes, but if desired the actual angular deiiectlon of the post can be calculated by trigonometry; in which casel a scale value oi 15/12 or 1.25 is taken as corresponding to the actual angular deflection. To ilnd vthis angle (in radians) one divides l.25 by the lens local length in millimeters.

Since one ray is undeviated, and since the higher order images extend from it in spaced series, there is no ambiguity as to which order o! 'Il I reflection the observer is utilizing. He merely counts from the undeflected image. This is an important feature of my system. In making use of multiple reflections for indicating, it is desirable that there should be no ambiguity as to which order of reilection the observed image belongs.

. While the system described is, in principle, op- I have a certain optimum value depending upon which order of reflected image is to be used. The following are the considerations by which the silvering to be applied to the plane faces vis Y determined.

' mirrors. by reason of their excellent optical prop Reference to Fig. 3 shows that to get n reflections from a moving mirror (where n means any integral number, e. 4g., 1. 2, 3, etc.) there must be n other reflections taken from a fixed mirror; also, that in the case of all images due to interreilection, transmission through two and only two mirror surfaces must occur. Thus to get maximum intensity in the 11th reflection. the mirror surfaces should be highly reflecting and only slightly transparent because there are 2n reections but inail cases only 2 transmissions.

The reflection and transmission characteristics of the opposed plane faces may be different, but

since experience shows that it is best to have the reflection and transmission factors of the two mirror-faces equal, only this case need be discussed in detail.

The intens'ty of the nth reflection In, depends on 2n reections but on only 2 transmissions. If the transmission of each mirror surface is t, then the reflection factor is (1 -t) andthe intensity can be represented as where L is the incident" light intensity. By known calculation methods it is determined that this function has a maximum value for n, expressed follows:

Thus in the case of the nth reflection the maximum possible light intensity is obtained when the transmission factor is 1:1/ (n+1). For example. usingl the 6th image, the optimum trans mission factor is 1/7. Using -the 10th image it is l/ll; in other words the transmission is 9 per cent.

Experience shows that on the whole the best practical results with the system are obtained when about the 10th order reflection image is used, the angular amplication in this case being l0 times that obtainable if only a single reflection were used. In this case a transmission of 9 per cent is, asstated. optimum, but results almost as good i image intensity not less than 90 per cent ot optimum) can be abtained with transmission within the range .12.5 and 6.3 per cent.

I regard aluminum alloy films as best for the ertles and their durability, lbut other materials,

Fig. 4 shows the slit as of greater width at one end than the other. While ordinary linear slits can be used, theform illustrated is advantageous in that the enlarged end provides some stray light which makes the scale or crosshairs readily visl ible.

The embodiment of the invention described is the simplest and is in many adaptations the' most useful. 'Ihe system is readily adaptable to all sorts of measuring instruments, the movable lens 19 being in all cases aixed to an angularly movable part of the apparatus. Certain modifications o'f the apparatus have utility in particular installations. Some of these are exemplified in Figs. 6-11.

The embodiment just described, and all other embodiments of the invention, have two partly transparent mirror faces and a projecting lens combination. The minimum number of optical components, therefore, is four; two mirror facings 20 and two lenses. These parts can be provided as individual optical parts or as combined optical' parts, i. e., the mirror facing being on the lens,

subject to a few simple restrictions. It is best to have the reflecting faces close together and the 25 light being multiply reflected back and forth between the mirrors should be essentially parallel light". The lenses can be double convex,v positive meniscus, or plano-convex; that is. of any converging type. The lenses can be simple 3 or highly corrected. The mirrors should be high quality optical flats and partly transparent. The mirror lms can be mounted on glass, quartz or other suitable backing. Since every air-glass interface imposesa 4 per cent (approximately) loss 35 of light intensity, there is some advantage in -combining the functions of the optical parts.

While it is not necessary to take great pains to conserve light intensity, 'a general simplification results in that fewer mechanical mountings are 40 required if the lenses are made plano-convex and the plane side is used for the mirror backing as well as just for the other side of a lens. Carried to the limit, the system then consists of two planoconvex lens-mirror umts requiring only two 4I mounts (Figs. 1-3). This system is ideal if one lens mirror is fixed and the other mounted on a moving part with a well dened axis. If, however, small translational displacements of the lensmirror occur, there will result small changes in the optic axis of the system. For use on portable instruments, such as a gravimeter which has to be leveled for each new station, the system shown in Fig. 6 presents advantages.

Fig. 6 shows a system similar to that of Fig. 2 55 but having two fixed plano-convex lenses |3i and l3i', one of which (advantageously the one on the eyepiece side) has a partially silvered plane mirror face 33, and a parallel plane-faced glass element 'l0 is arranged therebetween, element 10 00 being affixed to an angularly movable part (2B) of the measuring apparatus. The face 33 of element 'IB adjacent face 33 is silvered. Element 10 is thus an angularly movable mirror. This ar rangement gives results similar tothose obtained 65 with the device of Fig. 2.' l

'I'he embodiment of Fig. 6. I-regard as being on the whole the most useful'system. .The disassociation of the moving mirror fromv the lens on the source side of the system results in a simpliflcation oi' the manipulations involved in the initial adjustment of the system. The system is espe clally useful in gravimeters of the type indicated in Fig. l. Since the optical part attached to the moving system is essentially a plane-parallel 'Il plecenof glass withy a mirror surface on the veyepiece side, there occurs n'o shift of the optic axis Aot the system of the moving clement which swings v due to seismic disturbances. or is 'slightly misalined due to errors in leveling. In the Fig. -6

system, errors due to a lens not rotating about its nodal point are avoided. This system possessesl all the advantages of 'a system with four completely disassociat'ed optical elements with the added advantage ot one less mechanical mounting. since the mounting for the'xed mirror is likewise the mounting 4for the lens on the eyepiece side of the system.

The lenses of Figs. 1 to 3 and 6 can be of the same focal length or diierent.' It different. the image of the slit will be magnied or minified depending on the ratio of focal lengths. This is sometimes advantageous.

While the systems described are sumciently sensitive, even for very delicate apparatus, it is possible to 'secure twice the sensitivity by making use of a modiiicati'on, which is essentially a double system so constructed that two equal .and opposite series of images are produced upon angular deflection of the apparatus. between the 11th image of each series.

One such modification is shown in Fig. 'l (elevation) and Fig. 8 (plan). Two plano-convex lens combinations 1| and 1|', are ilxed to the angularly movable apparatus element in the aline- Iment shown, and these are opposed to two fixed lenses |3| and |3|' as shown. Plane faces 33'of lenses 1| and 1|', and piane faces 33 of lenses |3| and |3|', are silvered to serve as mirrors. This modification is shown as having line-iliament lamps 18 and 18' in lieu of the lamp-lens slit combination of Figs. l, 2 and 6. The various elements are arranged so that the primary filament images of the two systems coincide on thel screen, as at 60; To achieve this result in this modification, lens 13| is 'of slightly shorter focal length than lens 1|, and lens 1|' is of slightly longer focal length than lens |3I. If desired lens |3I' can be of the same focal length as lens 1|, provided lamp 18' is set back to bring it at the principal focus of lens III'. l

Uponangular deilection of the apparatus element, a'double series of images is produced as shown in Fig. 8. The series on the right of the reference point (60) is denoted by 6|, 62,`etc.,

and that on the left is denoted by 6|', 62', etc.

Measurement is conveniently made between the 10th image of each series. The sensitivity of this device is just twice that of the devices of Figs. 2

and 6 vhaving similar dimensions, because, when post 26 is rotated. the two sets of images move in opposite directions with respect to the central image. This causes the images to move through twice the distance in the field of view.

The moving element (lenses 1| and 1I') can be made by simply mounting two such lenses in a mount. or it can be made by cementing two thin plano-convex lenses to a fiat piece of glass.

Figs. 9, 10 and 1 1 show optional forms of lens combinations which achieve similar results.

Fig. 9 shows a combination of four half-lenses. the moving lens comprising oppositely disposed half-lenses 8| and 8|' as shown. and two iixed lenses 9| and 8|' being provided. This arrangement gives results similar to those achieved with the device oi Figs. 7 and 8, but only one light source is necessary. In using this embodiment a lamp-lens-slit assemblage as in Fig. 2 can be used, and the appearance of the image array is as in Fig. 8. Lens Il has a slightly shorter Readings are taken" focus4 than lens Il, and lens l8|' has a slightly longer focus than lens 9|, to image the source properly. 9|", and faces 33' of lenses Il and 8|', are par tlally-silvered to provide plane mirrors of senilQv circular shape onthe opposed faces as shown.A` Fig. l0 shows ,an arrangement of Afour fixed The planeiaces 32 of lenses 9| and lenses, lui., m, m ancm, and Aaimerait element consisting 'of a parallelogram-shaped glass block or double prism :|18 with partially transmitting plane mirror faceslvlitl and. Iii' 'opposed'v parallel' to lenses `lll'nnd V|2|, and

beveled,unsilver`ed"faces lill and |4|. The operation of this device is' similar to that Vof Fig. 6'. The reason .for beveling faces' |49 andl Hi lis to make them not parallel with respect to faces 33 of lenses I|| and |2I; if they .were parallel, spurious and disturbing reflection images might appear. The angle of Abevelis slight, only a few degrees oi of parallel being necessary to achieve the results. The bevel is exaggerated in the showing for the sake of clarity. arrangement a double lamp arrangement as in Fig. 1 is employed. and the aspect of the reiiec tion images is asin Fig. 8. The modication of Fig. l0 has the advantage that all four lenses can be of the same focal length if desired. The prsmatic element (|10) is disposed sothat the prisms bend the rays from the iilament, in a direction along the length of the lament; thus no diiiiculties 'are introduced due to dispersion bythe prisms. The image is not widened-across its width. but across its length, which is of no importance. A

Fig. 11 shows an arrangement analogous to Fig. 10 but havingonly two fixed lenses, |3| and |3|', and a parallelogram discoid element 21|. In some cases the bevel can be dispensed with entirely, element 21|| taking the form of a plane discoid similar tn that of the device of Fig. 6, but silvered in halLcircles on opposite sides. rather than over the whole face of the disc on only one side as in Fig. 6. In such modication it is advantageous to have the moving element slightly canted, i. e., oil' parallel, in the zero position. the left half only of lens 13|', are silvered, as at 33, to provide mirrors. This system, which utilizes a single lens-slit combination as in Fig. 2,

gives a double series of images as in Fig. 8.

These double-series systems are useful in instruments i'or measuring directly the tidal forces of the sun and moon. in which instruments extremely great sensitivity is essential.

Figs. l5 and 16 show a useful modiilcation of the invention, which may be used with all embodiments but which is illustra-ted'in connection with the system of Figs. l and 2. By mounting the movable mirror lens element (3|) on post 26 in such manner that the plane of themirror (33) makes an angle to the axis oi' rotation of the post, the multiple images produced on angular rotation of the post are no longer allned, but ap-4 pear in the eyepiecein echelon arrangement as indicated at R in Fig. 16. When the post swings back to central position. the images appear overlapped in a line. as indicated at C in Fig. 16.

As the post swings past center to the left. the

In this. .3

The right naif only or lens Isl. and

ilcation is useful in instruments based on the 75 .cating deflections in two dimensions, by sub- 1 Vstitutin'g for the slit an illuminated pinhole null principle. As the alined vimages partly overlap. there is less loss in light for any particular image.

In all embpdimentsof the invention the mirror faces of the various elements are advantageously 'disposed close together, and ordinarily the faces are parallel in the zero position.

The systems are shown as'adapted for measur ing deflections in one dimension only, but those ofv Figs. Zand 6 can be readily 4 adapted for indibalance comprises the usual elements; a base 2N,

upright 2li, knife edge support 2|2, beam 213, knife edge 2H and pans 215.v The moving plate 10 'of the optical system is aillxed to the beam, in alinernent with the knife edge, in a mount 2l! as shown'. The under side of plate 1li is silvered.

' Fixed lenses I3i and i3l' are attached to the upright, as shown, in mounts 2|! and 220. The upper side of lens |31' is silvered'vas described. A

straight filament lamp 18 is provdedas in Fig. '1. I

On the base are mounted an inclined Iscale-screen 22|, of ground glass or Celluloid. and an inclined mirror 222 arranged to reflect the measuring beam on to the scale (Fig. 13). The operation-of thisv device is similar to that of Fig. 6. The bal.

ance point is indicated by the series of images collapsing and. coinciding. The optical system forms an extremely'sensitive balance indicator for weighing apparatus.`

Fig. 14 shows a recording barograph in which the optical system of the invention is incorporated. A light-weight oat 240, consisting of a very thin blown glass shell filled with air under suitable pressure and hermetically sealed, is suspended inside a cylindrical shield 2li (to protect the shell from the effect of air currents and to provide air damping for the moving system) from a helical ribbon coil spring 242 arranged in a manner analogous to the gravlmeter spring of "Fig, 1. A small air gap 241 is left between the float and the shield' as shown. The spring is shown as having 'a blmetalllc .temperature compensating section 243. Changes in atmospheric pressure cause the float to rise or fall, changing the length of the spring accordingly. The change in length is accompanied by twisting of the spring. The angular deection is not great, but the optical indicating means are capable of accurately measuring it. The light source and lens combination is similar to that of Fig. i2, but in lieu of a scale there is shown a recording device comprising a cylindrical lens system 245 adapted. in cooperation with lenses i3! and I3I',.to forma point image of the lamp filament upon a moving tape 2&5 of sensitized film or paper. In operation. the tape is moved at uniform speed by suitable mechanism (not shown). Variations in atmospheric pressure are accompanied by twistings of 'the plate 10. A family ,of wavy curves is delineated on the moving tape. any one of which can be used as a basis of measurement. Should the curve used for measurement go oil the tape due to a great variation in atmospheric pressure, one of the other curves can be selected for use in measurement at this point, the appropriate factor being applied.

. It should 'be noted that vthe optical system pro-. vides indicating or recording means which im. pose no resistance upon the moving apparatus element. Indeed, the 'system has no mechanical connection whatever with the apparatus mecha.-

' In an optical system of the present invention l of sensitivity equal to that of an ordinary lamp-l4 mirror-scale system, the light pathv is only 1l as long, where n is the order of reflection utilized.

For example, in the system of Fig. 1 utilizing thev 10th reflection, a total light path of 2 feet is equivalent to an ordinary system having the lamp and scale spaced 20 feet from the mirror. In a system as in Fig. 2 with a source-screen distance of 22 inches and using a 10:1: ocular, it was`easy to measure angular deflections as small as 0.1 second of arc. In a system of similar dimensions but using only a single reflection, no angleless than 1 second can be measured with the same ease.

Using a straight line scale, deflections noted on the scale are practically a linear function of the angular deflection when the angular deflection is not more than a few degrees; the sca-le defiection being proportional to the tangent of the angular displacement. For larger ranges of deflections, a tangent correction can be applied arithmetically or, the scale can be curved about the center of the lens combination. For most installations the tangent correction can be neg' lected, as the total angular aperture o! the eyepiece is less than 1 degree of arc.

.l'o secure best results, in all cases the plane mirror surfaces should be finished optically fiat to la high degree of precision. In ordinary lens and mirror practice, surfaces flat to within M4,

where )i is the wavelength of light with which the lens or mirror is used, are considered perfect; that is, when the maximum difference in level between the hills and valleys" of the surface is vwithin M4. it is considered unnecessary to work for further flatness. However, in mirrors for the present apparatus. the surfaces should be finished to a higher degree of accuracy, since the same area of the surface is used several times, and error is cumulative. For best results the surfaces should be fiat to within x/r. where n is the order of reflection used. For example. using 1l) reflections. the mirrors ideally should be fiat to within or about 0.00004,mm. for yellow sodium light (D line). However, many of the advantages of the invention are secured when the precision of workmanship is much lower than this. In practice )./10 flats are used. This degree oi'r accuracy is advisable onlyl in the case of the surfaces between which multiple reflections occur. For other surfaces. such as the convex lens surfaces, conventional degrees of precision are suiiicient.

Except as otherwise indicated, the usual considerations applying to optical systems in general apply here. For example, the ratio of aperture to local length of the lenses determines the brightness of the reflection images; sensitivity is higher the higher the magnification of the ocular used. up to a 'certain limit: and the longer the light path (i. e.. the longer the focal length of the lens combination) the higher is the sensitivity. The lenses can be corrected for aberrations in known ways; though this is rarely nec essary.- The light path can be bent as by the prisms of Fig. l in known ways to llt the system knowledge and skill for satisfactoryl operation.v

10 Moreover the systems embodied ,in properly con- 'structed apparatus are rugged and do not tend toget out of adjustment. They are thus ideal for use in portable precision apparatus su'ch as gravimeters. What I claim is:

. 1. 1n measuring apparatus having an angularly deflectable member, optical means for giv- Aing a magnified indication of such deflection. comprising a pair of plane, partially transparent mirrors arranged substantially parallel to each other, one mirror being fixed and one beingfat- 'cached to said deilectable memberv so that upon deection of the deectable member the mirror attached thereto is moved out of parallelism with z5 respect to the fixed mirror, a source of light. means for directing a collimated beam of light from said source through said mirrors in a direction substantially at right angles to lche plane of the mirrors, and means for bringing said col- 80 limated beam to a focus at a focal plane after its passage through the mirrors, so that upon deflection of the deectable membenvinier-reiiectlons occur between the mirrors and a series of images of the source is formedat the focalA 8l plane. l

2. Inmeasuring apparatus having an angularly defiectable member, optical means for giving a magnified indication of such deilection, cornprising a pair of plane, partially transparent mir- 40 rors arranged closely adjacent and substantially parallel to each other, one mirror being xe'dand one being attached to said deectable member for motion therewith, so that upon deflection of the ldeiiectable member the mirror attached thereto is'moved out of parallelism with respect vto the xed mirror, a source of light, and a pair of converging lenses, one lens being on one side of the mirrors and at a distance from thesour sub.

stantlally equal to the principal focus oi' the lens,

w so that the lens directs a collimated beam of light through the mirrors, the second lens being on the other side of the mirrors and adapted to bring the collimated beam to a focus at a focal plane, so that upon deilection of the movable mirror, multiple reilections occur between the mirrors and a series of images of the source appears at the focal plane.

3. The optical system of claim 2 wherein the stationary mirror is carried on one of the lenses, so and the other lens is attached to the deilectable apparatus member, and the other mirror is carried on said attached lens.

4. The optical system oi claim 2 wherein both lenses are stationary. and the stationary mirror 5 is carried on one of the lenses.

5. An optical system adapted to give a magnied optical indicationy of angular deflection of a mechanism part which is deilectable about an axis. comprising narrow light-emitting means, 70 four partially transparent plane mirrors, two'of said mirorrs being attached to the movable mechanism part, facing on opposite sides of the said axis and displaced from each other along the direction oi the axis, two similar stationary mit rors each closely opposed to and facing one of* said vmovable mirrors, and converging lens means Y on each side of said opposed pairs of mirrors. the lens means being so arranged as to collimate light from the source, pass it as a beam through the 5 said'pairs of mirrors and focus said beam at a.v focal plane after passage through vthe mirrors,

' whereby 'upondeection of said movable mech- Xanism part, multiple reflections of co y' light occur between the fixed mirrorsand the lo' y movable' mirrors, givingrise at said focal plane to a double series of images of the source o! light. 6. An optical system adapted for indicating angular deection of an angularly movable ap paratus part, lenses, a partially transparent mirror nlm on the plane face of each lens, one of, said lenses being fixed and one being attached to said movable apparatus part, the lenses being arranged with said plane faces facing each other, a source of 2o light and a screen, the source and the screen be ing arranged so that light from the source, is collimated by one of said lenses, is passed be tween said mirrors as a substantially collimated beam and is focused by the other lens on the 25 screen.

'1. An optical system adapted for indicating angular deflection of an angularly movable mechanism part, comprising two fixed plano-convex lenses the plane facesof which are disposed fac- 3o.

ing each other, a partially transparent mirroi on one of said plane faces, a movable optical element positioned between said faces and having two plane faces each of which is opposed parallel to said lens plane faces, a second partially 35 transparent mirror on one of said plane faces of the optical element adjacent the rst mirror. said movable element being attached to said mechanism part, concentrated small-width means for emitting light and a screen, the lightemitting means and the screen being so arranged that light from the lightemitting means passe# through the lenses and the movable element and is focused on the screen. l

8. The optical system of claim 1 wherein the mirrors are very thin'alun'num alloy films o! 4" such' thickness as to transmit a small fraction of the light impinging thereon.

9. The apparatus of claim 1 wherein the source of light is an illuminated slit member, the slit being narrow at one end and wide at the other 50 to provide stray light.

10. In measuring apparatus having a member angularly deectable about an axis. optical means for giving a magnified indication of such deection, comprising a pair of plane, partially 55.

transparent mirrors arranged closely adjacent each other, one being parallel to the axis and the other inclined at a small angle with 'respect to the axis, one mirror being attached to said deflectable member and the other being xed, n 60.

small source of light, means for directing a collimated beam of light from the source through said mirrors and means for focusing the beam, after its passage through the mirrors, upon a focal plane, so that upon deflection o! the deflectable member an echelon series of images of the source is formed atthe i'ocal plane.

ll. The optical system of claiml wherein the transparency of each mirror is between 12.5 and 6.3 per cent, to secure an optimum image for 7 the image corresponding to ten interrefiections.

ARCHER HOYT.

comprising two planooonvex 15.

ii o 

